The present invention relates to a method and apparatus for detecting the occurrence of back corona, i.e. electric discharges in the dust precipitated on the collecting electrodes of an emission electrode system of electrostatic precipitators which have one or more separate precipitator sections and which are used for purifying flue gases from industrial plants. In such precipitators the degree of purification increases proportionately with an increasing power input under operating conditions during which no back corona occurs. Where the dust layer on the emission system has a sufficiently high resistivity, a locally occurring overstepping of a current value characteristic of the type of dust and the current operating condition may, however, cause discharging in the dust layer with a resultant lowering of the degree of purification. It is, therefore, of essential importance to be able to immediately detect the occurrence of back corona in order to control the precipitator section for optimum cleansing of the flue gases.
U.S. Pat. No. 4,390,835 disclose a method for detecting back corona based on changes in the slope of the current-voltage characteristic curve. According to this patent, the mean current is utilized as a function of the mean value of the precipitator voltage. Similarly, according to U.S. Pat. No. 4,311,491, the mean current is utilized as a function of the minimum value of the precipitator voltage. According to Danish Patent Application no. 5118/86, detection is made by comparative measurement over a predetermined time interval of mean voltage, mean current and mean power fed to the subject precipitator section.
In recent years it has become common practice to utilize, in addition to the ordinary or continuous DC-voltage supply, a so-called intermittent voltage supply to increase detection efficiency. For example, according to U.S. Pat. No. 4,410,849, the power supply to the high voltage transformer is interrupted periodically for a specific number of half-periods of the main frequency, i.e. The frequency of the AC main supply line. Another method based on intermittent voltage supply is disclosed by German Published Patent Application no. DE 3525557 wherein a measurement is made over four consecutive half periods of the frequency of the main supply, after the power supply has been deliberately interrupted.
It is, therefore, an object of the present invention to provide a method and apparatus for reliable detection of the occurrence of back corona for precipitator sections operating with either continuous or intermittent DC-voltage supplies based on measuring the precipitator voltage before and after each spark-over.